| Abstract
| - Competitive solvation of the excess proton in protonated mixed methanol−water clusters [H+(CH3OH)m(H2O)n, m + n = 4] has been characterized by vibrational predissociation spectroscopy in combination withdensity functional theory calculations. The solvation topology of the clusters can be classified as (1) theclosed shell, in which a hydronium ion H3O+ is fully solvated by three neutral molecules forming a completesolvation shell, and (2) the open chain, where the excess proton is tugged between two mixed subunits in alinear chain. The existence of these two types of isomer is verified from a close examination of the characteristicfree-OH and hydrogen-bonded-OH stretching modes in the spectra. It is found that sequential replacement ofthe water molecule in H+(H2O)4 by methanol redistributes the population between the closed-shell and theopen-chain isomers. While the excess proton is preferentially taken by methanol (instead of water) in thechain configuration, it can be either localized as CH3OH2+ or delocalized as CH3OH···H+···CH3OH at m ≥2, depending sensitively on the number of the methanol molecules and the symmetry of the cluster isomers.In contrast to that of NH4+(NH3)m(H2O)n, m + n = 4, previously studied, this work provides a clear pictureof competitive solvation of a charge between the constituent solvent molecules within a cluster.
|
